This application is a 371 of PCT/JP01/00636 filed Jan. 31, 2001.
The present invention relates to a dielectric ceramic composition of excellent dielectric characteristics in a high-frequency region, particularly such a composition exhibiting a high unloaded quality factor (hereinafter referred to as xe2x80x9cQuxe2x80x9d) and a small variation thereof, and to a dielectric resonator formed of the composition. The dielectric ceramic composition of the present invention can be used in dielectric filters, multilayer circuit boards, etc. for use in a high-frequency region.
Compositions represented by BaOxe2x80x94ZnOxe2x80x94Ta2O5 are known to be dielectric ceramic compositions which can be used in a high-frequency region. Such dielectric ceramic compositions for use in a high-frequency region must satisfy the following requirements:
(1) a high dielectric constant (hereinafter referred to as xe2x80x9c∈rxe2x80x9d)
(2) a small absolute value of the temperature coefficient (hereinafter referred to as xe2x80x9cxcfx84fxe2x80x9d) of resonance frequency (hereinafter referred to as xe2x80x9cf0xe2x80x9d); and
(3) a high Qu in a high-frequency region.
The BaOxe2x80x94ZnOxe2x80x94Ta2O5 dielectric ceramic compositions are oxides represented by the compositional formula of Ba(Zn⅓Ta⅔)O3 and have a complex perovskite-type crystal structure. These oxides are generally referred to as BZT. The BZT dielectric ceramic compositions exhibit excellent dielectric characteristics, such as a high Qu. However, in recent years, there is a demand for a dielectric ceramic composition having a higher Qu, since the frequency region where such dielectric ceramic compositions are used has become higher; i.e., from the microwave region to the sub-millimeter region.
Publication of Unexamined Patent Application No. Hei 11-71173 discloses that a dielectric ceramic composition of more excellent dielectric characteristics can be obtained by incorporating a K component and a Ta component into an oxide represented by the compositional formula of Ba(Zn⅓Ta⅔)O3. Although incorporation of these specific components attains enhancement of dielectric characteristics, uniform Qu cannot always be attained, and variation in Qu may occur. Thus, provision of a dielectric ceramic composition exhibiting a smaller variation in dielectric characteristics is desired.
The present invention has been accomplished in order to solve the aforementioned problems, and an object of the invention is to provide a dielectric ceramic composition capable of attaining a high Qu without variation, by means of employing specific proportions by amount of elements in the predominant component formed of an oxide containing Ba, Zn, and Ta and by limiting the ratio by amount of K component to Ta component other than the predominant component and the ratio by weight of K to Ta. Another object of the invention is to provide a dielectric resonator formed of the dielectric ceramic composition.
The dielectric ceramic composition of the present invention contains Ba, Zn, and Ta, and is characterized by comprising 100 parts by weight of a predominant component represented by xBaOxe2x80x94yZnOxe2x80x94(xc2xd)zTa2O5 (x, y, and z represent compositional proportions by mol and satisfy x+y+z=1), wherein x, y, and z fall within a quadrilateral region formed by connecting points A (x=0.503, y=0.152, z=0.345), B (x=0.497, y=0.158, z=0.345), C (x=0.503, y=0.162, z=0.335), and D (x=0.497, y=0.168, z=0.335) (sides AB, BD, DC, and CA being included) as shown in FIG. 1; 0.2-1.6 parts by weight K as reduced to K2O; and 0.7-8 parts by weight Ta as reduced to Ta2O5, wherein the ratio by weight of K to Ta falls within the range of 0.185-0.4.
The dielectric resonator of the present invention is characterized by being formed of the above-described dielectric ceramic composition of the present invention.
In the dielectric ceramic composition of the present invention, the aforementioned xe2x80x9cxBaOxe2x80x94yZnOxe2x80x94(xc2xd)zTa2O5xe2x80x9d serving as the predominant component is an oxide having a complex perovskite-type crystal structure. Some portion of Ba atoms are substituted by K, and the perovskite-type crystal structure is thought to be maintained.
In the predominant component, when any of x, y, and z is in excess of the upper limit or less than the lower limit, variation in Qu becomes large, even though excellent ∈r and xcfx84f and a high average Qu are obtained. Thus, a dielectric ceramic composition of dielectric characteristics with small variations cannot be produced. When at least two of x, y, and z are in excess of the upper limits or less than the lower limits, the average Qu is prone to decrease and variation in Qu becomes large, even through ∈r and xcfx84f do not decrease. Preferably, by controlling x, y, and z so as to fall within a quadrilateral region formed by connecting points Axe2x80x2 (x=0.503, y=0.154, z=0.343), Bxe2x80x2 (x=0.497, y=0.160, z=0.343), Cxe2x80x2 (x=0.503, y=0.161, z=0.336), and Dxe2x80x2 (x=0.497, y=0.167, z=0.336) (sides Axe2x80x2Bxe2x80x2, Bxe2x80x2Dxe2x80x2, Dxe2x80x2Cxe2x80x2, and Cxe2x80x2Axe2x80x2 being included), excellent ∈r and xcfx84f can be maintained, and the average Qu can be further increased with further decreased variation, to thereby provide a dielectric ceramic composition of dielectric characteristics with small variations.
When the amounts of xe2x80x9cKxe2x80x9d and xe2x80x9cTaxe2x80x9d incorporated in addition to the predominant component are less than the above-described lower limits as reduced to K2O and Ta2O5, respectively, particularly when the amount of K is less than the lower limit, based on 100 parts by weight of the aforementioned predominant component, the resultant composition is difficult to sinter, and in some cases, sintered products cannot be yielded. When the amounts of K and Ta are in excess of the upper limits, Qu greatly decreases, and variation in Qu increases.
When the ratio by weight of K to Ta is in excess of the upper limit or less than the lower limit, Qu greatly decreases, and variation in Qu further increases. By controlling the ratio by weight of K to Ta preferably to 0.25-0.35, more preferably 0.25-0.30, excellent ∈r and xcfx84f can be maintained, and the average Qu can be further increased with further decreased variation, to thereby provide a dielectric ceramic composition of dielectric characteristics with small variations.
K and Ta other than Ta contained in the predominant component form an oxide having a perovskite-mixture-type crystal structure represented by KpTaOq. In KpTaOq, K is considered to occupy the Ba site of the predominant component, and Ta is considered to occupy the Zn site or Ta site of the predominant component. The dielectric ceramic composition of the present invention has a perovskite-type crystal structure formed from the predominant component and the perovskite-type structure of KpTaOq partially occupying the predominant component, and the entire perovskite-type crystal structure is considered to be a complex perovskite-type crystal structure. In the predominant component, atoms of at least one of Zn and Ta may be substituted to some extent by an element such as Mg, Zr, Ga, Ni, Nb, Sn, or a rare earth metal element; e.g., Y. These elements are readily substituted by Zn or Ta, maintain the perovskite-type crystal structure, and do not impair excellent dielectric characteristics. In the case in which some portion of Ba atoms are substituted by Sr, xcfx84f can be modified while the Qu value is maintained.
The dielectric ceramic composition of the present invention can be produced by mixing together oxides of Ba, Zn, Ta, and K or compounds other than the oxides of Ba, Zn, Ta, and K, which compounds yield corresponding oxides by heating; shaping the resultant mixture; and firing at 1300-1700xc2x0 C. In addition to the aforementioned oxides of essential metallic elements, there may be incorporated oxides of at least one element such as Mg, Zr, Ga, Ni, Nb, Sn, or a rare earth.metal element; e.g., Y. Through this incorporation, there can be obtained a dielectric ceramic composition in which atoms of at least one of Zn and Ta constituting the predominant component are substituted to some extent by at least one element of the aforementioned elements.
When the firing temperature is less than 1300xc2x0 C., a sintered product of sufficient density cannot be obtained, resulting in insufficiently increased Qu in some cases, whereas when the firing temperature is in excess of 1700xc2x0 C., potassium ions are readily eliminated through volatilization, and the surface of the sintered product becomes porous, resulting in a tendency of failure to attain sufficient improvement in Qu. The firing temperature is preferably 1350-1650xc2x0 C., particularly preferably 1400-1650xc2x0 C. In order to attain densification, the firing temperature is preferably controlled to 1500xc2x0 C. or higher, particularly preferably 1550xc2x0 C. or higher. No particular limitation is imposed on the firing time, and a firing time of 1 to 8 hours, particularly 2 to 6 hours, can be employed. Firing can be performed in an oxidizing atmosphere such as the natural atmosphere or a reducing atmosphere containing a small amount of hydrogen.
After completion of firing, the fired product may further be heated at a temperature lower than the firing temperature by approximately 50-250xc2x0 C. in an oxidizing atmosphere for 12 hours or longer, to thereby produce a dielectric ceramic composition of excellent dielectric characteristics with further lower variation. When the temperature of this heat treatment is excessively high, coarse grains are readily formed during grain growth, to thereby fail to provide a sintered product of uniform quality in some cases, whereas when the temperature of the heat treatment is considerably low, the crystal structure does not assume a superlattice structure of a long period, resulting in a tendency of failure to attain sufficient improvement in Qu. The heat treatment temperature is lower than the firing temperature preferably by 70-200xc2x0 C., particularly preferably by 70-170xc2x0 C., more preferably by 80-150xc2x0 C. For example, by controlling the heat treatment temperature to a temperature lower than the firing temperature by about 100xc2x0 C., a dielectric ceramic composition having a superlattice structure can be produced easily.
The atmosphere of the heat treatment may be an oxidizing atmosphere such as the natural atmosphere. The natural atmosphere is preferred in that no special operation or apparatus are needed. However, by elevating the partial pressure of oxygen in the oxidizing atmosphere to a pressure higher than that in the natural atmosphere, a dielectric ceramic composition having a more excellent Qu can be obtained. Thus, from the viewpoint of dielectric characteristics, an oxidizing atmosphere of an increased partial pressure of oxygen is preferred. The heat treatment is preferably performed for 12-20 hours. When the heat treatment time is excessively short, formation of a superlattice structure is difficult, resulting in insufficiently improved Qu in some cases. A heat treatment time of 15 hours or longer, particularly 18 hours or longer, will successfully attain intended effects. Heating for 24 hours suffices for the heat treatment, and no longer heat treatment is necessary.
The dielectric ceramic composition of the present invention is endowed with excellent dielectric characteristics; i.e., there can be attained a xcfx84f of xe2x88x9215 to +15 ppm/xc2x0 C., particularly xe2x88x9210 to +10 ppm/xc2x0 C., further xe2x88x925 to +5 ppm/xc2x0 C. When a test piece having a diameter of 16 mm and a height of 8 mm is formed from the composition, the test piece can be endowed with a product of the measured frequency and Qu of 25000-30000 GHz, wherein Qu is measured at a frequency of 4-6 GHz through a parallel plate dielectric cylindrical resonator method (TE011 mode). In addition, variation in Quxc3x97f0 (measured frequency) is very small, and there can be attained a standard deviation "sgr"nxe2x88x921, calculated by the below-mentioned equations, of 700 or lower, particularly 500 or lower, further 300 or lower.
The dielectric resonator of the present invention is characterized by being formed of the dielectric ceramic composition of the present invention, and is endowed with excellent dielectric characteristics. Specifically, a Qu, as measured through a reflection method at a resonance frequency of 1900 MHz, of 40000 or higher can be attained.
In the dielectric ceramic composition of the present invention, no clear reason for enhancement in Qu through substitution of Ba by K is elucidated. However, it is considered that one possible reason is that KpTaOq having a perovskite-type crystal structure forms a solid solution with the predominant component having a complex-perovskite-type crystal structure, to thereby provide a superlattice structure of a long period in the crystal structure of the dielectric ceramic composition. When KpTaOq of an unspecified composition is present, vacancies are regularly arrayed, to thereby form a superlattice structure. In addition, it is also considered that the presence of vacancies facilitates transfer of ions, elements, and the like during a firing step, to thereby promote densification. Thus, although conventional dielectric ceramic compositions of this type require long-term firing, firing by ultra-high-speed temperature elevation technique, etc. so as to attain densification, the dielectric ceramic composition of the present invention can be readily densified.